Lithographic processes are used in the manufacture of semiconductor devices, such as integrated circuit chips. The processes typically involve the steps of depositing a layer of a photoresist material, e.g., one which will react when exposed to light, onto a device substrate, such as a circuit chip wafer, by means such as spin-apply; baking the device, typically at a temperature of approximately 110.degree. Celsius (C), although high bakes can occur at temperatures above 140.degree. C., to set the photoresist layer,; selectively exposing portions of the photoresist layer to light, e.g. ultraviolet (UV) light, or other ionizing radiation, e.q. X-rays and ion or electron beams; and developing the photoresist layer by washing with a basic developer solution, e.g. tetramethylammonium hydroxide (TMAH), thereby removing the non-irradiated (as in a negative photoresist) or the irradiated (as in a positive photoresist) portions of the photoresist layer. This process, along with typical photoresist materials are described in Semiconductor Lithography, W. Moreau, Plenum Press, 1989, which is incorporated herein by reference.
The photoresist material is formulated by dissolving a polymer resin, a photoacid generator and a cross-linker in a casting solvent. A photosensitizing additive, commonly referred to as a sensitizer, is added to the formulation to increase the photosensitivity of the photoresist formulation. By varying the amount of sensitizer added to the photoresist, the photospeed of the development process can be modulated.
An important technical limitation of existing sensitizers is that they are not highly soluble in photoresist casting solvents or developer solutions, and consequently, the concentration of sensitizer that can be employed in the photoresist formulation is limited. Secondly, existing sensitizers are susceptible to sublimation during the baking process, thereby depleting the photoresist formulation of sensitizer. In addition, the sublimed sensitizer can coat the baking tools and then flake off during the subsequent process, redepositing on the device, resulting in further problems in the system.
While various sensitizers have been developed for use in photoresist formulations, the problems caused by their low solubility in photoresist casting solvents and developer solutions have not been addressed.
For example, U.S. Pat. No. 4,371,605, issued Feb. 1, 1983 to Renner and assigned to E. I. DuPont de Nemours and Company, discloses several anthracene derivatives that may be used as a sensitizer in the formulation of a photopolymerizable composition. The patent does not, however, address the problems caused by the sensitizers' low solubility.
U.S. Pat. No. 5,296,332, issued Mar. 22, 1994 to Sachdev et al. and assigned to International Business Machines Corporation, discloses high-sensitivity, high contrast, heat-stable photoresist compositions for use in deep UV, i-line e-beam and x-ray lithography. While the compositions disclosed are aqueous base developable, the potential problems caused by the sublimation of sensitizer and the deposition of the sublimed sensitizer as a physical precipitate on the semiconductor device remain unresolved.
Therefore, there exists a need to develop a sensitizer that is highly soluble in photoresist casting solvents and developer solutions, allowing the sensitizer to be used in greater concentration in the photoresist formulation, without the sublimation of the sensitizer during the baking process and the subsequent precipitation of the sublimed sensitizer during the development process.